Vehicle braking systems either of the hydraulic or air type, such as are commonly utilized and employed in automotive vehicles, generally comprise a master brake cycliner or valve which is interconnected in fluid communicating relationship with brake actuating cylinders mechanically coupled with the brake shoes at each respective wheel. In its simplest form, a fluid brake system merely comprises a single actuating fluid line interconnecting a master brake cylinder with the brake actuating cylinders. In a basic hydraulic fluid system, both master brake cylinder and brake actuating cylinders are cylinder and piston units with the pistons mechanically coupled, respectively, to either a brake pedal or to the brake shoes. Accordingly, operation of the brake pedal moves the master cylinder piston and thereby displaces and pressurizes the fluid through the brake system. Each of the brake actuating cylinders, coupled with the brake shoes at the respective wheels, will thus be operated through displacement of the pressurized actuating fluid from the master cylinder. A fluid reservoir is generally associated with the master brake cylinder to provide any necessary make-up fluid that may be required to replace system losses.
A substantial disadvantage of such a conventional basic hydraulic brake system is that it is unable to accommodate or compensate for a massive leak or rupture of the actuating fluid conduits interconnecting the master cylinder with the brake actuating cylinders. A substantial leak in the system, such as a rupture of the fluid line, results in a total failure of the brake system. In an automotive vehicle such as the usual family type car, a secondary or auxiliary brake system is normally not provided, and failure of the brake system is, in effect, a complete loss of braking capability. The only emergency brake system available is the so-called parking brake which is relatively ineffective in decelerating a fast moving vehicle.
Other types of vehicles through necessity resulting from type of use, such as school busses, may be provided with a reasonably effective emergency or alternate braking system. An alternate braking system may comprise a mechanical linkage to secondary braking shoes or an over-ride with respect to the brake shoes that are normally actuated by hydraulic cylinders. While the concept of such an alternate or emergency brake system appears to effectively attain the objective, there is an inherent disadvantage in that alternate mechanical brake systems require substantially greater efforts to operate than a hydraulic or air system. Also, an alternate system requires a substantial amount of time to operate as the operator must switch from one to the other and this is a particularly undesirable distraction from driving activities at a very critical time. There is a substantial time lag from the time the operator has ascertained that a primary fluid actuated brake system has failed and that it will be necessary for him to slow down and stop the vehicle through operation of a mechanical or other alternate system.
There has been some attempt to provide an alternate hydraulic braking system to perform the function of an emergency brake system. An example of such prior patented attempts is exemplified in U.S. Pat. No. 3,015,522 issued to R. B. Spikes on Jan. 2, 1962. In the system of the Spikes' patent, the master brake cylinder may be selectively and alternatively connected in fluid communicating relationship with dual brake actuating cylinders in the respective wheels by means of a solenoid actuated valve. This valve is actuated by an electric solenoid which is energized by means of an electrical switch operated by a mechanical linkage between the brake pedal and the master brake cylinder. Operation of the selector valve to interconnect the alternate hydraulic circuit with the master brake cylinder is determined by the degree of travel of the brake pedal. If the brake pedal is displaced beyond a predetermined point, it actuates the electrical switch thereby energizing the electrical solenoid and operating the selector valve to interconnect the alternate hydraulic circuit. While this system does provide an alternate hydraulic circuit arrangement for the purpose of safety, system operation is predicated on a physical displacement of the master brake cylinder beyond a predetermined point. If the brake system needs servicing to bleed air, the brake pedal may be sufficiently displaced to operate the electrical switch even though the alternate brake system is not required. If the system is losing fluid or has a massive rupture in the lines, the master brake cylinder will first displace a substantial amount of the fluid in the lines through the rupture before the apparatus senses this condition and operates the selector valve. As a consequence, the master brake cylinder may not have an adequate quantity of hydraulic fluid with which to then operate the brakes through the alternate system.